Compute pairs of colnames based on the following architecture:
Suppose a dataframe with column ordered as follow : c("A", "B", "C", "D")
The pairs will be computed as follow
AA AB AC
BA BB BC
CA CB CC
In order to to construct a full pair matrix. This function has to be called
for each part (lower corner, upper corner, diagonal). The call on diagonal should be last
in order to have access to neighbour subplots metadata.